Hugo wrote:
> Situation is: comparing three of my lenses with fixed aperture (let's
> say: 5.6), the light meter gives on all three of them a different
> exposure time. That is: 50mm OK, 85mm +1 stop, 135mm +1/2 stop.
> After extensive comparisons in which all possible other explanations
> were ruled out, it left me with the only remaining cause: the 'pen' inside
> the mount of the lens that mechanically tells the camera what aperture
> is momentarily in use is not in proper place in two of the lenses.
> Now here's my question: is it possible to open up the rear of the lens
> myself, adjust the relative position of the 'pen' a little and at the
> same time be reasonably confident to end up with a working lens?
OH NOOOOOO!! I don't think you want to do that. I can think of a couple of
extra possibilities that might give you this result. One possibility is
that you might have differences in the coatings in these three lenses,
and/or a different number of air/glass surfaces inside. A second
possibility is that there may be different levels of cleanliness on the
surfaces of each of the lenses. Either of these possibilities would cause
differences in the amount of light transmitted at a given aperture. A third
possibility is that even if there are the exact same number of elements in
each lens, with the exact same coating, and all of the lenses are absolutely
clean, you might still have differences in light transmission due to
manufacturing variability. Finally, a fourth possibility is that you are
metering on a scene which is not evenly illuminated. The longest
focal-length lens, for instance, might be looking at a small area with a
much different average reflected intensity than the shortest focal-length
lens. Wait until an hour or so before sunset, then try metering each lens
by focusing on infinity and pointing straight up into a clear blue sky.
I personally wouldn't worry if the total difference is only one f/stop, but
I might want to shoot some test shots of some step tablets or other contrast
target, then compare the negatives. Hope this helps.
Bill Peifer
Rochester, NY
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